STRONG DEEP RECIRCULATIONS IN A 2-LAYER WIND-DRIVEN OCEAN

The circulation of a geostrophic, two-layer ocean with Ekman layers at the top, the bottom, and the interface and driven by an anticyclonic wind stress is determined by numerical integration. Inertial effects a nd lateral friction are ignored. A nominal ratio of upper- to lower-la yer thicknesses of 0.15 leads to the formation of a region in the nort hwestern comer of the basin with lower-layer geostrophic contours that originate from and return to the western boundary layer. This region is referred to as unblocked. Elsewhere in the interior the lower layer is quiescent and the system behaves like a reduced-gravity system. Th e smallest amount of friction that could be treated by the variable gr id procedure that was used leads to a western boundary layer with a wi dth that is 0.1% of the width of the basin. For this limiting case flu id in the lower layer of the unblocked region flows out of the western boundary and turns southward and then southwestward to return to the western boundary layer as a relatively concentrated jet near the south ern edge of the unblocked region. The characteristic width of that jet , in which friction is not negligible, is about 3% of the width of the basin. In the upper layer there is a jet of comparable thickness that flows northeastward out of the western boundary layer into the interi or of the basin. This jet is above the lower-layer one, and the total, barotropic transport obeys the Sverdrup balance (required by the baro tropic vorticity equation). Thus, the model exhibits separation of the western boundary current from the coast even when the interface does not outcrop. That this separation occurs only for a system with very l ittle friction may be why it has not been obtained in OGCMs, which ten d to be rather viscous. Two approximate, analytical models are propose d. One is semiempirical and makes use of a linear approximation to the calculated scatterplot of potential vorticity and pressure in the low er layer of the unblocked region. The other is based on the assumption that the width of the lower-layer jet approaches zero as bottom frict ion diminishes toward zero. The main features of the circulation that was obtained numerically are captured by these approximate analytical models. A comparison with the results of the quasigeostrophic treatmen t of Ierley and Young is also given. The present analysis supports the Ierley-Young conclusion that the western boundary layer does not just respond passively to the interior flow but that it affects the interi or flow directly.